1. Field of the Invention
The present invention relates to an LED driving circuit, and more particularly relates to an LED driving circuit with low dimming protection blocking function.
2. Description of Related Art
FIG. 1 is a schematic diagram of a conventional LED driving circuit. The LED driving circuit comprises a converting circuit 130, a current regulator and a converting controller 120. The converting circuit 130 is coupled to an input power source Vin to convert the input power source Vin into a driving voltage Vout for lighting an LED module LM. The current regulator comprises a current regulating controller 100, a transistor M and a current detecting resistance R. A first end of the transistor M is coupled to an end of the LED module LM and a second end thereof is coupled to the current detecting resistance R. A current flowing through the LED module LM also flows through the current detecting resistance R, so as to generate a current detecting signal Cs indicative of the current of the LED module LM. The converting controller 120 is coupled to a connection node of the transistor M and the LED module LM and controls a power conversion of the converting circuit 130 according to a voltage feedback signal VFB of the connection node.
The current regulating controller 100 comprises a controller 102, an open-circuit comparator 104, a counter 115 and an OR gate 110. The controller 102 generates a current control signal Ga according to the current detecting signal Cs and a reference voltage signal Vr to regulate an on-resistance of the transistor M, thereby stabilizing the current flowing through the LED module LM at a predetermined current value. The converting circuit 130 can not provide the current flowing through the LED module LM when the LED module LM is open-circuit. A voltage level of the current detecting signal Cs is down to 0V and so is lower than a voltage level of an open-circuit judgment voltage Vop at this time. The open-circuit comparator 104 generates a high-level signal when the voltage level of the current detecting signal Cs is lower than the voltage level of the open-circuit judgment voltage Vop. The counter 115 is coupled to a counting capacitance Ct and executes time counting when receiving the high-level signal generated by the open-circuit comparator 104 for avoid an erroneous judgment due to a dimming signal or other noises. The counter 115 outputs a blocking conduction signal Fa when the time period of the open-circuit comparator 104 generating the high-level signal is longer than a predetermined period of the counter 115. The OR gate 110 is coupled to the counter 115 and receives a dimming signal Dim. The OR gate 110 regulates the current controlling signal Ga according to the dimming signal Dim for conducting and stopping to conduct the current flowing through the LED module LM. When the counter 115 generates the blocking conduction signal Fa, the controller 102 turns off the transistor M to stop lighting the LED module LM.
When the converting circuit 130 and the converting controller 120 operate normally and the dimming signal Dim is at low dimming, the voltage of the connection node of the transistor M and the LED module LM will be increase to a higher voltage level. Then, the converting controller 120 judges that the driving voltage Vout is over-high and then decreases the voltage level of the driving voltage Vout. Under this situation, the voltage level of the current detecting signal Cs is still lower than the voltage level of the open-circuit judgment voltage Vop even though the transistor M is turned on according to the dimming signal Dim afterward. The current regulating controller 100 judges incorrectly that the LED module LM is open-circuit and then executes a latch protection. Therefore, the conventional LED driving circuit has a problem of not starting and operating when the dimming signal is at low dimming or a noise are erroneously judged as a dimming signal. Even after the dimming signal is turned to be at high dimming, the LED driving circuit still cannot be restored to operate due to the latch protection.